Worked on the qmk/qmk_firmware repository to enhance build reliability by addressing a cross-platform path resolution issue in build_keyboard.mk. Focused on Makefile scripting and build system configuration, the developer delivered a targeted bug fix that ensures USER_PATH is resolved correctly on case-insensitive filesystems such as macOS and Windows. By guarding the wildcard check with a non-empty QMK_USERSPACE, the solution prevents incorrect absolute path resolution when usernames match home directory names. This change reduced user confusion and support requests, demonstrating careful debugging and an understanding of filesystem nuances while maintaining minimal risk and ensuring compatibility across different operating systems.